P h yeieal and chemical properties of
PURE FLUOROCARBONS R. D. Fowler. J. \I. Hamilton. Jr.l. J. S. Kasper?. C. E. Weber?, W.R. Rurforcl 111. and H. C. Anderson3 THE JOHYS H O P I t I l S UUIVERSITY, BILTIhIORE, \ID.
THE Tapor presbures o f pure perfluoro-n-butane, perfluoroniethjlc\ cloheuane, perfluorodiniethj lc) clohexaiie, and perfluoro-n-heptane were determined. Heats and entropies of \aporization were calculated. Critical constants of perfluoro-n-butane, perfluoro-n-heptane, and perfluoromethjlc>clohevane were e\ aluated, and boiling points, densities, refractiie indices, surface tensions, and \iscosities are gi\en for a number o f pure fluorocarbons. The behaiior of some fluorocarbons towards hj drolj sis w a s also studied.
F T E R the preparation of a number of pure fluorocarbons by the method of synthesis described in previous papers of the current series (I), a demand for the determination of some of their phy,Qicaland chemical characteristics arope. I t is the purpose of this paper t o present determinations of vapor pressure, critical constants, boiling points, densities, refractive indices, surface tensions, viscosities, and behavior towards hydrolysis of some pure fluorocarbons. VAPOR PRESSURE
The vapor pressures of pure perfluoro-n-butane, perfluoro-nheptane, perfluorotnethylcyclohexanc,, and perfluorodimethylcyclohesane vere measured. SAMPLES.The compounds studied were obtained by purification and fractionation of crude produced in this laboratory by the CoF3 process. The perfluoro-n-heptane was first distilled in a ten plate column in this laboratory, then successively fractionated in thirty and sisty-nine plate columns in the laboratory of A. L. Henne a t The Ohio State University {There, in addition, i t mas further purified by fractional crystallization. The perfluoromethylcyclohexane and perfluorodinicth-lcyclohexane samples Tvere purified in the same manner, except that the final crystallization step v a s omitted. Although the perfluoromethylcyclohexane could be shown to be a pure compound, the Ride boiling range (0.9" C.) of the purest sample of perfluorodimethylcyclohesane strongly suggested that it was a mixture of isomers. This is consistent with the nature of the starting material, bis(trifluoromethyl)benzene, n-hich is reported t o be such a mixture, predominantly ineta and para; the nornial ortho component of commercial xylene is thought to be destroyed during the preliminary substitution process. Other work in this laboratory shon-ed that the isomer ratio should be preserved in the mixture of 1,3- and 1,4-bis(trifluorometh\-l)benzeneThen it is treated with CoF3. The perfluoro-n-butane sample was also produced in this laboratory, but no low temperature facilities n-ere available and it was fractionated a t the Esso Laboratories. They reported the boiling point to be - 1.4 = 0.2' C. a t 769 nim. T h e best values of the boiling points for all the samples are listed in the following table. The boiling point of the perfluoron-butane was corrected t o standard pressure, based on the vapor pressure data. The values for the others were determined ' Present address, E. I. du P o n t de Senlours 8i Company, Inc , \\-ilniingt o n , Del. Present address, General Electric Company, Sc,henectady, S . T. Present address, Socony-Tacuum Oil Company, Paulaboro, S . J.
by I,. Henne with an accurate boiling point apparatus;. The refractive indice? and den=itic.s, n-hrire availablc, are included as indications of the purity of the compounds. Cnnipound
PerRuoro-ri-butane Perfluoroniethylryclohexane Perfluoro-n-heptane PerHuorodinietliylcyclohesane
n . I).. c .
drl
-1.7*0.2 i 6 83
....
82.47 101,5-10? 4
1.7996 1 7333 1.8580
....
1 2'816 1,2617 1.2902
;IPPARATTS A X D TECHSIQCE. The isoteniscopc of Smith and Llenzies (21 nxs modified slightly t o include a n additional bulb and n-a': connected by means of a manifold to vacuum and pressure outlets and to a mercury manometer. The other leg of the manometer v a s krpt at a pres-ure less than 10-2 mm. of mercury by pumping to eliminate the effects of atmosphei,ic pressure variation. The additional bulb was included to permit degassing (removal of permanent gases) 11-ithout loss of sample. The liquid was dried over P?05,sealed in the first bulb, and chilled with liquid nitrogen. The system n'as evacuated, and the sample b o i l d gently under vacuum into the adjacent bulb where il was recondensed with liquid nitrogen. The measurements were made i n the usual manner with a cathetometer to read the height of the mercury column. During the niea;urements the temperature of the thcrmostat bath was maintained to t.0.02" C. I n determining the vapor pressure of pcrfluoro-n-butane, the pressures werc measured by Bourdon-tube gagcs having pressure ranges of 30 inches vacuum to 50 pounds per square inch, and 0-400 pounds, reqpectively. The system consisted of two gages connected through a manifold to a reservoir containing the sample, a \racuuiii outlet, and a perfluoro-n-butane storage cylinder. The system was enclosed in an air thermostat (temperature deviation, 1 0 . 2 ' C J , while the liquid reservoir could be immersed in a liquid thermostat (temperature deviation, 10.02" C,). I n the experiments a portion of the perfluoro-n-butane was distilled into thc liquid reservoir under vacuum and was degassed by repeatedly chilling the sample with liquid nitrogen and cvacuating until no permanent gas could be detccted. T o ensure sufficient liquid throughout the crprriment, the ;imount vas noted from the loss in m i g h t of the supply cylinder. hfter degassing, t,he liquid reservoir was surrounded with the thcsrnioststing liquid, and the equilibrium vapor pressures read. T o prevent condensation through the system, the air thermostat was regulated at least 10" C. higher than the liquid temperature. RESTLTS. The vapor pressures of perfluoro-n-heptane, perfluoromethylcyclohesane, and pcrfluorodimethylcyclohexane were studied using the isoteniscope, n-hich allowed direct and accurate measurements. -2 pressure range of approximately 10 to 1000 mm. mercury was covered, and the method was found t o be twth rapid and reproducible. Table I and Figure 1 give the corrected data. Figure 1 shows that all the experimental points fell on a straight line, as did the boiling points of perfluoro-n-heptane and perfluorodiniethylcyclohesane (indicated by triangles), whereas the 1 boiling range for perfluorodiniethylcyclohesane falls across and t o one side of the line. The degree of accuracy of the measurements was estimated from a critical study of the data and experimental conditions. O
316
INDUSTRIAL AND ENGINEERING CHEMISTRY TABLE I. 1-APOR PRESSTRE~
Perfluoro-nheptane T:mp., Pressure, C. mm. H g 2.70 25 2 37 4 10.01 20.58 64 1 31.23 108 3 40.04 157 7 49.65 233 9 344 8 59.81 489 4 69.60 80 0 693 85.2 818 93,s 1054
Perfluoromethylcyclohexane T%mp., Pressure. C. mm. H g 5.85 43.9 21.74 94.6 30.04 138 4 39.18 202.5 49.83 305.4 59.79 437, 615. I 69.91 721.4 74.97 846 80.0 87.2 1047
3
Perfluorodimethylcyclohexane Tzmp., Pressure, C. mm. Hg 2.89 12.3 1l.E 19.7 20,81 31.4 30.04 49.3 39 92 ,8.4 49.93 121.1 s9. so 180.7 69.71 262.5 80.1 ., 3m, ,m 90.1 524 95.' 612 100.1 8 12 106.8 854 114.5 1053
1'erHuoro-n-
butane __ _~__ Teomp.,
Pressure.
C.
dtm.
3 0 0 1 0 9 3 0 5 0
-28 -20 0 10 20 29 40 50 70 65 70 82 90
2
1
column of Table I (for pri.>aures lrss than 50 pounds per square inch I :>re acCurate to 1'; : the 1iighc.r pressurri are accurate to approximately 2rc. The (lata are given t o the last significant figure consistent with the accuracy of the method. S o correction for variation in the atmospheric pressurc' \vas required, since it n-a-: eGvntially constant a t 760 nim. Y
0.348 0,500 1.11 1 02 2.29 3.14 4.21 5.6 7 2 8 0 9.2 12.2 13.7
4
Vol. 39, No. 3
HEAT O F VAI'ORIZ.ATIO\
Figures 1 and 2 shon- that the csperimental points fell on straight lines; yincc: thc plots shon-ed little or no change i n *lope over the temperature rang(' COIIsidcrcd, tlic, heats of vaporization ~ v e r ee:rlculated using t hc intc grated (.'lapeyron-('l:tu.-ius equation. These values and the corresporidiug entropics [if vaporization (Trouton's constant) are given in Tahlr: 11. From thv values of entropy, these compounds did not apprxr to tie high1.i. associatd.
T.~BI.E 11.
HEIT- h S D E:XTROPIES O F ~ . X P ~ J R I Z a T l O S
Heat of Vaporiznti on, Cal I l l o l e
Conipound Perfli~oro~~~etl~ylcycloheuane I'erfluoro-ii-heptane I'erfluorudiniet hylcycliihexane Perfluoro-n-butane
7830 8240 85S0 5580
C n t rop5- ,o i Vaporiza t i u ~ i , Cal Mole-Degree 22 4 23.2 .>,> -_
'J
20.8
CRITIC.AL C O S S T A X T S
Experimentally, the vapor pressures were reproducible t o =to.1 mm. as read from a mercury manometer, and these values Tvere corrected for the relative density of mercury under experimental conditions compared to the standard temperature of 0" C. S o correction TT-as necessary for the variation in gravitational constant. T h e temperatures were read on mercury thermometers, which were calibrated against a National Bureau of Standards platinum resistance thermometer. The tabulated values are accurate to =!=0.02"C. over the range 0-80" C. and t o *0.06" C. a t higher temperatures. T h e corrected values of the temperature are given to the nearest 0.01' over the lower temperature range and the nearest 0.1" over the higher temperature range; the pressures are given to the last significant figure consistent with the temperature. Since the vapor pressure of perfluoro-rr-butane was much greater than that of the higher boiling liquids over the same temperature range, the isoteniscope could not be used: the measurements were made by the simpler but less accurate method using Bourdon gages. The data obtained are presented in Table I and Figure 2. Here again, within the accuracy of the method the esperimental points fell on a straight line. However, the boiling point of this sample ( - 1.4" C.), based on a distillation carried out in another laboratory, does not lie on the curve: this indicates a somewhat lower value ( - 3 " C.), Corrections were applied to the experimental temperatures and pressures in the usual manner. T h e mercury and pentane thermometers were calibrated against a platinum resistance thermometer, and, although the temperatures were known to +=0.05° C., they are reported to the nearest 0.1" C. to be consistent with the pressure values. When the gages were calibrated against, a mercury manometer and a hydraulic gage tester, the low pressure gage was found to be the more accurate. The values in the last
The samples u5cd i n the determination of the various critical constants were portions of those previou,4y descritied. The critical temperatures were nieasurcd using 1-nun.-bore thickwalled glaus capillaries, and the samples were (leg manncr before sealing. The measurements were made i n a rapidly stirred oil bath, and the temperature could be held conutant to 0.1" c. The orthobaric den>ities xere determined in a similar manner using 2-mm. bore capillaries n-hose volume had been dctermined a t 1-em. graduations by weighing with mercury. The total volume was corrected for the cone formed upon sealing, and a millimeter scale was attached to allow interpolation between the graduations. CRITICALTEMPERATURE. T h e critical temperatures of perfluoro-n-butane, perfluoro-n-heptane, and perfluoromethylcyclohexane 17vei-e taken to be the points a t which the liquid menisci ,"-
70 50 4030 r 20 -
-0
e
2 ??
a
-
IO
7-
5 4321.0 0.7 0.50.4 a3 0 o.2 .I
~
270
290
310
330
Temperature (f x Figure 2.
350
370
390
18)
Vapor Pressure of Perfluoro-n-butane
371
INDUSTRIAL AND ENGINEERING CHEMISTRY
March 1947
BOILING POINTS, REFRACTIVE INDICES, AND DENSITIES
disappeared i n capillary tubes containing the liquid in the presence of its vapor only. The critical pressures were not experiment,ally determined, but approximate values were calculated from the vapor pressure-t,emperature relations, assuming the heat of vaporization t o be constant over the entire range. The values are given in the folloning tahlc: Tc, ' C. Perfluoro-n-butane Perfluoro-n-heptane Perfluoromethylcyclohelane
Pc,
The samples on which these properties were measured were the purest produced in this laboratory. The purification of the perfluoro-n-hept,ane, perfluorodimethylcyclohexanc,and perHuoromethylcyclohexane in the laboratory of A. I,. Henne has already been discussed. The other compounds shown in Table 111 were purified in this laboratory as discussed in a previous paper ( I ) . The absolute accuracy of the boiling points as determined by d. L. I-Ienne is *0.01" C. DenPities were determined with pycnometers i n the usual manner; their absolute accuracy is +0.0002. Refractive indices were obtained ivith a n Xhbe refractometer of >peck1 design. capable of measuring values a s l o a as 1.12. Thc absolute accuracy of the n~easurenicnti? *0.0004. The results are given in Tat& 111. From these data it wau possible to c:alculatc the specific refractivity, r , of each compound from t l v Lorentz-Lorenz equation arid use the resulting figure to determine a value for A l t ~for perfluorocarbons. I n this second calculation Eisenlohr's v:tlue of 2.418 \vas used for the atomic refractivity of carbon. Tahk: 111 gives the spccific refractivities calculated from tlie experimental data, together with the resulting value of .1Rr. The last column consists of the theoretical specific refractivities calculated using this value of -1Itb and 1:iscnlohr's valuc~of ARr. The cliangc CJf refractive index with teinperat urc \vas measured for perfluoro-n-heptane by determining the refractive index at 5' intervals from 10" t o 30" c'. I t was found that a plot of these points closely approximated a straight line with the value of dn/ clt = -0.0004/"
..ita1
213 16 24
113.3 202 ,5 "13 4
The critical points could l x reproduced to =0.1" C., upon repeated heating and cooling: the ahsolute values of the teniperatures are accurate to +0.2" C. a': determined hy calihration of the tlicrmonieters. The calculated crit ieai pressures are nece3sarily inaccurate since the heat of vaporization must decrease to zero a t t h e critical point: thep are included as approximate values which should be upper limits for the prewurcs.
.
95 90
2
f
85
2
80
&
75
a
e.
70
SURFACE TENSION AND
65 KO
40 0.0
The surface tensions of a fc\v purc materi:ils were measured with a du SoYiy interfacial tensiomctcr, and the results were used t o calculate parachor values. These resultq, a s ell as the atomic parachors obtained from thc,ni, follow: 02
06
04
08
1.0
1.2
Interfarial Tension a t 200 c.
Density, gJcC. Perfluoro-ri-heptane l'erfluororiiethylcyclohexane Perfluoro-p-dimethylcyclohexanr
Figure 3. Orthobaric Densities of I'er0uoro-n-butane
45.0 55.0 65.0 75.0 85.0 113.3
1.41 1.36 1.32 1.26 1.19 0 63'3
0.053 0.063 0.084 0.110 0.147 0.635
Diol 46
m
=
+
d ~ t q ~d i v v
Since two tubes were used, the densities can be evaluated. The temperatures were found to be exact to the nearest 0.1' C., and the apparent volumes were reproducible upon heating and cooling.
T ~ R L111. E '
Compound Perfluorodirnethylcyclopentane Perfluoroethylcyclopentane Perfluororneth ylcyclohexane Perfluoro-n-heptane P erfluoro-p-dimethylcyclohexane Perfluoro-m-dimethylcyclohexane Perfluoro-o-dirnethylcyclohexane Perfluorohexahydroindane Average ARF
Viscosity, Sec. 37.80' C. 9 8 . 9 0 ° T
Yiscosity Index
..
30.0
.....
35.0 56.7 1861.0
69:6
-122.0
980 0
57.2
-104.2
,.
, . . . .
.....
-
1.24
0.001.
PHYSIC.4L cOXSTAXTt.
R.P., 0
c.
71.67 75.05 76.32 82.43 100.97 102.12 102.61 117.83 I'erfluoro-l,3,5-trimethylcyclohexa1 125.18 a
,i
24.8 24.6 24.4
STABILITY TOWARD HYDROLY~IS. .bong the quantitative chemical experiments made with fluorocarbons mere measure-
Extrapolated critical densiry
The determination was made using calibrated glass capillaries containing a known mass rn of sample. The volumes of the liquid, V L , and the saturated vapor, vi, \yere noted; if d L and dv are the corresponding densities, it follows that
c.
ti^,^, 0 a t 10 Mrn.
